The present invention relates to a thickness-width shear quartz crystal vibrator suitable for use in an electronic wrist watch.
Recently, wrist watches have been using electronic components and thereby the timekeeping accuracies of these electronic wrist watches have been greatly improved. The flexural quartz crystal vibrator tuning fork has been used as a time-standard of the said electronic wrist watch. However, since the frequency-temperature characteristic of said flexural quartz crystal vibrator tuning fork has a quadratic characteristic curve, it is difficult to get highly accurate and stable resonance frequency which covers a wide temperature range. Therefore, by means of performing temperature compensation by using a barium titanate condenser, the electostatic capacitance thereof varies according to temperature, and quartz crystal wrist watches having passably high accuracy have been entering into practical use.
But, when the higher accuracy of a quartz crystal wrist watch is desired, it is necessary to adjust the temperature characteristics of both temperature the compensating condenser and quartz crystal vibrator to the optimum point. Moreover, it is impossible to increase the accuracy over a certain limit because the capacitance of the temperature compensating condenser varies with the lapse of time. Accordingly, an AT-cut quartz crystal vibrator, the frequency-temperature characteristic thereof having a cubic characteristic curve, has been drawing attention as a possible solution to the above problem, and the research and development thereof are continuously going on so as to be able to use it for wrist watches, however, it cannot be used as yet for wrist watches.
The AT-cut quartz crystal vibrator being developed has been a plane and rectangular element and both ends of the vibrator in the length direction have been finished into bevel shapes, and the electrodes for exciting have been provided on both top and bottom horizontal surfaces of the AT-cut quartz crystal vibrator. When an alternating voltage is impressed between the said electrodes for exciting, thickness-width shear vibrators can be maintained. Further, the distribution of polarization is to be taken into consideration in order to understand the vibrating modes of said rectangular AT-cut quartz crystal vibrator. When "p" indicates the number of crests of a sine waveform of polarization directed for its electrical axis (lengthy direction), "q" indicates the number of crests of a cosine waveform of polarization directed for its thickness and "r" indicates the number of cosine waveform of crests of a polarization directed for its width perpendicular to the said electrical axis. Thus, each vibrating mode can be indicated with (p, q, r).
The said rectangular AT-cut quartz crystal vibrator vibrates with the modes (p, q, r) being (1, 1, 0), so that if the rectangular AT-cut quartz crystal vibrator is miniaturized with said vibrating modes, it cannot be used for wrist watch since a number of spurious vibration are generated. Further, as for the vibrator of this type, it is theoretically impossible to vibrate with the vibrating modes (1, 1, 1) because each electrode is provided on the top and bottom surfaces in the thickness direction of the vibrator. Actually, however, the vibrator is able to be excited with vibrating modes (1, 1, 1) because the distribution of polarization thereof is not perfectly symmetrical. However, because of the characteristics of this vibrating mode, especially, since the equivalent impedance thereof is large and the Q-factor thereof is low, the said vibrator cannot be used for wrist watches.